The present invention relates to a dog clutch construction for a transmission device, and more particularly relates to such a dog clutch construction which can aid in avoiding the canting of a sleeve member thereof, as the dog clutch is engaged or disengaged. In particular, the present invention is suitable for application to a transmission system for a vehicle of the type in which, selectively, either the front wheels only can be powered, for use of the vehicle in relatively good road conditions such as on a paved road, or alternatively all four wheels can be powered, for use of the vehicle in relatively poor road conditions such as off the paved road surface. In such a vehicle, a power switching and transfer device is required which can switch the vehicle between single axle operation--i.e. front wheel drive only type operation--and double axle operation--i.e. front and rear wheel drive type operation or four wheel drive operation. Such a power switching and transfer device typically includes a dog clutch construction for engaging and disengaging the supply of rotational power to the rear wheels, and the problem of canting of the sleeve member of the dog clutch can become acute.
In detail, such a power switching and transfer device typically includes a power output shaft for outputting rotational power to the rear wheels of the vehicle, via a propeller shaft and a differential and rear axle assembly and so on, when and only when the vehicle is to be operated in four wheel drive mode, with the rear end of the power output shaft connected with regard to rotation to the front end of said propeller shaft, and a power input shaft the front end of which is supplied with rotational power from the main transmission of the vehicle which powers the front wheels thereof. These power input and power output shafts are typically mounted within a casing with their inner ends close to one another, and have dogs (i.e. toothed wheels) mounted or formed on said inner ends, these dogs being closely approached and opposed to one another. Further, typically a dog clutch sleeve member is generally formed as a hollow cylinder with its inner diameter substantially the same as the outer diameters of the toothed dogs, and is further formed on its inner surface with teeth corresponding in number and spacing to the teeth on the toothed dogs. This sleeve member is fitted over the toothed dogs so as to be slidable thereon in the axial direction between two extreme axial positions, in one of which extreme positions this sleeve member is located over and is rotationally engaged to only one of the toothed dogs, and in the other of which extreme positions the sleeve member is located over and is rotationally engaged to both of the toothed dogs. Further, a dog clutch drive member is typically provided, which can be driven to and fro (as for example by a piston of a hydraulic cylinder--piston assembly) in the axial direction of the power input shaft and the power output shaft for selectively either engaging the power input shaft to the power output shaft via the first and second toothed dogs and the sleeve member, or disconnecting the power input shaft and the power output shaft from one another, and this drive member is connected to the sleeve member by a means for doing so in such a way that they are kept substantially fixed with respect to one another with regard to movement along the axial direction of the power input shaft and the power output shaft while being allowed to rotate substantially freely with respect to one another around the axial direction.
Now, this sort of typical construction is subject to a considerable problem, because the means for connecting the drive member to the sleeve member has heretofore been such a construction as a fork member or shift fork, and since such a construction bears on the sleeve member at only two points of its circumference (typically diametrically opposite points, in fact), therefore a great risk has existed of forcible canting of the sleeve member, since inevitably a certain amount of play exists in the mounting of the sleeve member over the toothed dogs, and accordingly angling of the axis of the sleeve member with respect to the axis of the power input and output shafts is possible. In the case that such a fork member is used for shifting the sleeve member, when considerable force is thus being exerted on the sleeve member as for example when the teeth of the sleeve member are first being engaged with the teeth of the second dog (which are not engaged therewith when the transmission device is in the disengaged condition), then the sleeve member may very probably twist or cant with respect to the dogs, thus causing jerky engagement of the dog clutch construction and grating of the dogs and the sleeve member. This can also cause undue slowness of engagement of the dog clutch construction, and/or shock in the drive train of the vehicle incorporating it. Further, such a grating will naturally lead to quick wear on the dogs and on the sleeve member, which will cause the transmission to have an unduly short service life, and in the extreme case may even lead to premature failure thereof. Yet further, this problem tends to become accentuated by a feedback process, because such canting of the sleeve member while it is being slid on the dogs and especially while high torque load is being transmitted by the dog clutch causes quick wear on the dogs and on the inner part of the sleeve member, thus causing the abovementioned play therebetween to become greater, which in its turn allows more extreme canting of the sleeve member relative to the dogs.